In recent years, in consideration of the environment, synthetic resin emulsions used in a wide variety of industrial fields including, for example, automobiles, electrical and electronics, and architecture are converted into aqueous compositions. The resins obtained from such resin aqueous emulsions have a defect in that, when exposed to light containing ultraviolet rays such as sunlight or heat, they undergo deterioration or discoloration due to the ultraviolet rays or heat, thus causing a decrease in molecular weight which in turn causes a decrease in strength.
To improve the durability (light resistance) of the resin against ultraviolet rays or heat, an antioxidant is usually added to a resin aqueous emulsion (see, for example, JP 7-138522 A, etc.) Examples of the known antioxidant include 2,6-di-t-butyl-4-methylphenol, 1,3,5-trimethyl-2,4,6-tris-(3,5-di-t-butyl-4-hydroxybenzyl)-benzene, n-octadecyl-3-(3′,5′-dibutyl-4′-hydroxyphenyl) propionate.
However, those conventional antioxidants are powdery and have poor affinity for the resin and water in the emulsion, so that addition of them to aqueous emulsions of resins causes various disadvantages. For example, the antioxidants are poor in dispersibility, which results in sedimentation of the antioxidants with lapse of time. This decreases the homogeneity of the resin aqueous emulsion and the mechanical strength of a coating or film formed therefrom.
To eliminate the defect as mentioned above, isooctyl-3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate, which is a liquidized preparation of the above-mentioned antioxidant, has been proposed. Use of this can considerably improve the dispersibility in the resin aqueous emulsion. However, even so, the emulsification and dispersion requires a large amount of an emulsifier, and it can be easily expected that this will give an adverse influence on the water resistance of the resultant coating. Also, there is a concern about bleeding out of the antioxidant with lapse of time.
An object of the present invention is to provide a resin emulsion having the function of oxidation inhibition and a resin aqueous emulsion having the function of oxidation inhibition which have the following properties: they exhibit excellent dispersibility when added to an emulsion of another resin or an aqueous emulsion of another resin; they improve dispersion stability of the emulsion or aqueous emulsion of another resin; in a resin obtained through addition of them; the resin having the function of oxidation inhibition does not deteriorate desirable properties of the other resin; they impart the other resin with excellent light resistance; they involve little bleeding out of the resin having the function of oxidation inhibition on the surface of the coating made of the resin obtained after the addition; and they impart the resin obtained after the addition with prolonged stable light resistance and further impart the other resin with excellent alkali resistance and solvent resistance when the other resin has poor alkali resistance and poor solvent resistance. Another object of the present invention is to provide a resin emulsion and a resin aqueous emulsion obtained by adding, to an emulsion of another resin and an aqueous emulsion of another resin, the resin emulsion having the function of oxidation inhibition and the resin aqueous emulsion having the function of oxidation inhibition, respectively. Another object of the present invention is to provide a resin having the function of oxidation inhibition.